Optical delay lines are an essential part of most time-resolved optical experiments, including time-domain terahertz technology, ultrafast optics research, time resolved detection, interferometric spectroscopy, optical coherence tomography, most optical pump/probe experiments, and other applications. Optical delay lines generally employ beam splitting optics to duplicate a pulse of light whereby one copy of the pulse is sent via a first optical path through one part of a system and the second copy is sent via a second optical path through a second part of the system that incorporates an optical delay arrangement such that the length of the second optical path can be changed in a controlled manner. A common optical delay technique reflects pulses of light off a moving retro-reflector mirror that is mounted on a motorized translation stage, such as a linear screw type translation stage, or on voice coils. Another technique is to simply stretch the optical fiber through which the pulses of light travel.
U.S. Pat. No. 5,220,463 to Edelstein et al. describes an optical delay line with opposite-facing hollow front surface retroreflectors that are offset to each other. A standard mechanical translating device that is connected to one of the retroreflectors adjusts the distance between the retroreflectors along a line of movement that is parallel to the reflected light beam as it enters and exits the retroreflectors. In one variation, a movable retroreflector is mounted on a linear slide that is constrained for movement in a straight line on a stage. A motor driven drive wheel links an eccentric pivot on the drive wheel with a pivot on the movable retroreflector. As the wheel rotates, the retroreflector moves back and forth in a generally sinusoidal fashion with respect to the stage so that the rotational motion of the wheel is translated into a linear motion. This optical delay line arrangement, which requires a relatively massive mirror to constantly stop and accelerate, is not suitable for applications that require both high amplitude and frequency.
One such application involves online measurements using terahertz (T-ray or THz) radiation, which lies on the boundary of electronics (millimeter waves) and photonics (infrared). The terahertz spectrum encompasses the wavelengths approximately in the range of 3 mm to 15 μm. Terahertz radiation exhibits a large range of modifications on passage through varying materials or on reflection from materials. Such changes include attenuation or partial attenuation of different frequencies of the waveform and other alteration of the waveform depending upon the material through which the radiation or pulses pass. Terahertz radiation interacts strongly with polar molecules, a prime example being water. Water molecules absorb terahertz waves, on the one hand limiting penetration of the radiation in moist substances, and on the other hand making it readily detectable even in very low concentrations. It can be used for detecting low concentrations of polar gases. However, terahertz radiation will penetrate non-polar substances such as fats, cardboard, cloth and plastics with little attenuation. Materials including organic materials have varying transmission, reflection and absorption characteristics to terahertz radiation. Accordingly, use of terahertz radiation can indicate the presence of different materials.
Typically, a terahertz time-domain spectroscopy setup has three major categories of components: optics components include the laser and optical-delay line; terahertz components include the emitter and detector; and control components that are used to modulate terahertz generation, synchronize the delay line, and perform data acquisition. Both the optical-delay and the optical modulator impose limits on the overall speed of the system. In a delay line used in terahertz time domain spectroscopy, the magnitude of the path length change affects the frequency range over which a measurement can be obtained and the repetition rate generally governs the time it takes to scan a frequency window. Higher repetition rates lead to more measurements per time period.
Since most moving displacement designs (other than fiber stretching) as exemplified by U.S. Pat. No. 5,220,463 operate on the principle of linear displacement of a mirror, conventional optical delay arrangements do not generate both high repetition rates and large displacements due to the high acceleration required. The art is in need of an optical delay system that affords both large amplitude and high frequency. In particular, commercial online scanning measurement systems would benefit from an optical delay configuration which can provide large displacement with a repetition rate that is faster than that which is currently available.